1. Field of Use
Hydrostatic drive trains used as power modules for front-end loader type vehicles typically include a variable displacement pump coupled with an output drive motor, the pump having an infinitely variable position swash plate which controls the power output of the drive motor and is movable from a central neutral position to a full forward or a full reverse position with an infinite range of power outputs between the neutral position and the respective full power positions. However, because of the dynamics of the flow of hydraulic fluid within a hydrostatic drive system, it is difficult to achieve an unaided central neutral position. Without an appropriate centering mechanism the swash plate of the hydrostatic pump would not automatically remain in the neutral position when returned there, but rather the dynamics in the fluid flowing through the system would cause the vehicle to "creep" in either a forward or reverse direction.
2. History of the Prior Art
Patents have generally been directed to various proposals for centering devices. For example, a typical spring arrangement may be found in U.S. Pat. No. 3,488,955 entitled "Hydrostatic Transmission Drive Pump Swash Plate Control" wherein a torsion spring biases a cam follower against one side only of a cam slot to prevent the cam follower from hunting between two sides of the cam so that a neutral position of the drive pump swash plate is established whenever the control lever is moved to its neutral position.
In U.S. Pat. No. 3,477,439 entitled "Riding Mower with Forward and Reverse Belt Drive", a bell crank is pivoted about a bolt in response to a forward force applied by a connecting rod which is connected with a pedal. A single torsion spring in contact with the bell crank returns the bell crane from a forward position to an initial position.
A third form of centering device available in the prior art may be found in U.S. Pat. No. 3,313,174 entitled "Self-Centering Speed Control for Hydraulic Transmissions" in which a captured spring unit operates to return a control lever to a neutral position. Although several other arrangements could be cited here, nowhere does the prior art disclose the present invention.